In LTE (Long Term Evolution) system, an RACH (Random Access Channel) uses the cyclic shift sequence of Zadoff-Chu (ZC) sequence as a preamble, such a cyclic shift sequence can also be called as a ZCZ (Zero Correlation Zone) sequence.
In an actual system, after being powered on, a terminal first performs a downlink synchronization, and then detects BCH (Broadcast Channel) A BS (Base Station) informs the terminal, through the BCH, a cyclic shift step length NCS and a first ZC root sequence index used for the RACH in the current cell, the terminal takes advantage of a certain mapping rule to calculate the sequence number of the corresponding ZC sequence according to the index, then generates a usable ZCZ sequence as the preamble sequence according to the cyclic shift step length NCS and a certain “cyclic shift limitation rule”. If the number of the preamble sequence is less than a threshold M, the terminal automatically increments the ZC sequence index, and continues to generate a ZCZ sequence by using the next ZC sequence until the total number of the ZCZ sequences is bigger than or equal to the above threshold. Finally, the terminal randomly selects one sequence from all the usable ZCZ sequences generated as the preamble and transmits it.
For a ZC sequence with a sequence length NZC, (NZC−1) different root ZC sequences may be obtained, wherein the uth root ZC sequence is defined as
                    x        u            ⁡              (        n        )              =          ⅇ                        -          j                ⁢                              π            ⁢                                                  ⁢                          un              ⁡                              (                                  n                  +                  1                                )                                                          N            ZC                                ,          ⁢      0    ≤    n    ≤                  N        ZC            -      1.      The length of the ZC sequence is the length of the preamble sequence. In the LTE system, the length of the ZC sequence is 839, each cell has 64 sequences for the preamble, these 64 sequences may be different cyclic shifts from a same root sequence, or may be different cyclic shifts from different root sequences. For the uth root sequence, its cyclic shift sequence is xu,v(n)=xu((n+Cv) mod NZC), wherein Cv is the cyclic shift amount, mod represents the remainder after an integer is divided by another integer. In the LTE, the cyclic shift used for generating the preamble sequence can be classified into a restricted set and an unrestricted set, wherein the restricted set (i.e., with cyclic shift restriction, which can also be called as a high speed set) considers the frequency offset effect resulted from high speed and can ensure that there is no overlap between the search windows of respective preamble sequences, but the cyclic shift amount Cv used for generating the preamble sequence will be restricted; the unrestricted set (i.e., without cyclic shift restriction, which can also be called as a normal set) is applicable to the situation with little frequency offset, and the cyclic shift amount generated by its preamble sequence will not be restricted, and the value of the cyclic shift which generates the preamble is Cv=v·NCS, wherein v=0, 1, 2, . . . , └NZC/NCS┘−1, └x┘ represents the maximum integer which is smaller than or equal to x, i.e., the floor operation of x. It can be seen that in the situation that there is not any cyclic shift restriction, the value of the cyclic shift is restricted by the cyclic shift step length, for example, according to the formula of the cyclic shift of the LTE unrestricted set, if NCS is 0, └NZC/NCS┘ is of no consequence.